U.S. patent number 7,324,465 [Application Number 10/399,330] was granted by the patent office on 2008-01-29 for random access channel access apparatus for mobile satellite communication system and method therefor.
This patent grant is currently assigned to Electronics and Telecommunications Research Institute. Invention is credited to Soo-Young Kim, Ho-Jin Lee, Kwang-Jae Lim, Moon-Hee You.
United States Patent |
7,324,465 |
Lim , et al. |
January 29, 2008 |
Random access channel access apparatus for mobile satellite
communication system and method therefor
Abstract
The present invention relates to Random Access Channel (RACH)
access apparatus for mobile satellite communication system and
method therefor. The method for accessing random access channel
(RACH) on satellite system, random access channel (RACH) carrying
message from a plurality of mobile stations to the satellite
system, the method includes the steps of: receiving preamble and
the message, the message successively transmitted with the preamble
from the plurality of mobile stations; and transmitting acquisition
response signal corresponding to the preamble or the message to the
plurality of mobile stations. Accordingly, success of packet
reception of satellite system is improved and transmission delay is
reduced.
Inventors: |
Lim; Kwang-Jae (Taejon,
KR), Kim; Soo-Young (Taejon, KR), Lee;
Ho-Jin (Taejon, KR), You; Moon-Hee (Taejon,
KR) |
Assignee: |
Electronics and Telecommunications
Research Institute (KR)
|
Family
ID: |
19693868 |
Appl.
No.: |
10/399,330 |
Filed: |
October 17, 2001 |
PCT
Filed: |
October 17, 2001 |
PCT No.: |
PCT/KR01/01746 |
371(c)(1),(2),(4) Date: |
April 15, 2003 |
PCT
Pub. No.: |
WO02/39622 |
PCT
Pub. Date: |
May 16, 2002 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040014452 A1 |
Jan 22, 2004 |
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Foreign Application Priority Data
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Oct 17, 2000 [KR] |
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2000-60961 |
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Current U.S.
Class: |
370/278; 370/347;
370/329 |
Current CPC
Class: |
H04B
7/18558 (20130101); H04W 74/006 (20130101); H04W
74/0833 (20130101); H04W 84/06 (20130101) |
Current International
Class: |
H04Q
7/00 (20060101) |
Field of
Search: |
;370/310,316,336,441,447,508,328,329,278,341,337,347,442,335 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2346779 |
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Aug 2000 |
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GB |
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2001-69576 |
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Mar 2001 |
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JP |
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2001-204072 |
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Jul 2001 |
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JP |
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1999-84349 |
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Dec 1999 |
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KR |
|
2000-14424 |
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Mar 2000 |
|
KR |
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2000-38285 |
|
Oct 2000 |
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KR |
|
Primary Examiner: Pezzlo; John
Attorney, Agent or Firm: Blakely, Sokoloff, Taylor &
Zafman
Claims
What is claimed is:
1. A method for accessing a random access channel (RACH) through
which a plurality of mobile stations transmit messages to a
satellite access network, comprising the steps of: a) receiving a
preamble and a message transmitted successively from the mobile
station; and b) transmitting an acquisition indicator (AI) signal
for the preamble and the message to the mobile station.
2. The method as recited in claim 1, wherein a transmission time
unit of the preamble and the message is an access frame of a
predetermined length on the RACH selected independently by each of
the mobile stations.
3. The method as recited in claim 2, wherein the access frame is
time-aligned with a radio frame of a control channel, which is
broadcasted from the satellite access network in conformity with a
starting point of the radio frame that each of the mobile stations
receives.
4. The method as recited in claim 2, wherein the preamble and the
message are transmitted based on a starting point of a sub-access
frame selected independently by each of the mobile stations among a
plurality of sub-access frames, the access frame being divided into
a plurality of sub-access frames which have the same length.
5. The method as recited in claim 4, wherein the preamble and the
message are transmitted after a transmission time offset selected
independently by each of the mobile stations from the starting
point of the access frame or the sub-access frame.
6. The method as recited in claim 1, wherein the Al signal is any
one of: a positive AI signal informing of an acquisition indication
of the preamble or the message; and a negative AI signal informing
that the use of the RACH through which the preamble or the message
is transmitted is not permitted currently.
7. The method as recited in claim 6, wherein the positive AI signal
includes a positive acquisition indication (AI) value.
8. The method as recited in claim 6, wherein the negative AI signal
includes a negative acquisition indication (AI) value.
9. The method as recited in claim 1, wherein said method for
accessing the RACH is performed in a physical layer of the
satellite access network.
10. The method as recited in claim 9, wherein the physical layer
function is located in an earth station of the satellite access
network.
11. The method as recited in claim 9, wherein the physical layer
function is located in a satellite of the satellite access
network.
12. The method as recited in claim 1, wherein the preamble includes
N number of sub-preambles generated by a signature code of a set of
signature codes used for the RACH to which the mobile station tries
to access, N being an integer larger than 2.
13. The method as recited in claim 12, wherein the preamble
includes: N-1 number of first sub-preambles having the same code;
and a second sub-preamble which is a conjugate code of the first
sub-preambles.
14. The method as recited in claim 12, wherein the preamble
includes: N-1 number of first sub-preambles having the same code;
and a second sub-preamble with an inversed sign of the first
sub-preambles.
15. The method as recited in claim 12, wherein the preamble is
composed of a continuum of the N number of sub-preambles.
16. The method as recited in claim 12, wherein the preamble is
composed of the N number of sub-preambles apart in a predetermined
interval intermittently.
17. The method as recited in claim 4, wherein the preamble and the
message are generated by a spreading code corresponding to the
selected sub-access frame in the RACH.
18. The method as recited in claim 2, wherein the access frame is
larger than a maximum difference of round trip delay times between
the different mobile stations and the satellite access network.
19. The method as recited in claim 5, wherein the access frame is
larger than a sum of a maximum difference of round trip delay times
between the different mobile stations and the satellite access
network, and the twice of a maximum transmission time offset.
20. The method as recited in claim 4, wherein a length of the
sub-access frame is the same as that of the radio frame broadcasted
from the satellite access network on the control channel.
21. The method as recited in claim 2, wherein the AI signal for the
preamble or the message is transmitted based on the access frame
periodically.
22. The method as recited in claim 1, wherein the AI signal for the
preamble or the message is transmitted on a time basis of a radio
frame broadcasted from the satellite access network on a control
channel.
23. A method for accessing a random access channel (RACH) through
which a plurality of mobile stations transmit messages to a
satellite access network, comprising the steps of: a) transmitting
a preamble and a message successively to the satellite access
network; b) receiving an acquisition indicator (AI) signal for the
preamble or the message from the satellite access network; and c)
retransmitting the preamble or the message based on the AI signal,
or waiting for a response for the message.
24. The method as recited in claim 23, wherein a transmission time
unit of the preamble and the message is the access frame of a
predetermined length on the RACH selected independently by each of
the mobile stations.
25. The method as recited in claim 24, wherein the access frame is
time-aligned with a radio frame of a control channel, which is
broadcasted from the satellite access network, in conformity with a
starting point of the radio frame that each of the mobile stations
receives.
26. The method as recited in claim 24, wherein the preamble and the
message is transmitted based on a starting point of a sub-access
frame selected independently by the mobile station among the
plurality of sub-access frames, the access frame being divided into
a plurality of sub-access frames which have the same length.
27. The method as recited in claim 26, wherein the preamble and the
message is transmitted after a transmission time offset selected
independently by the mobile station from the starting point of the
access frame or the sub-access frame.
28. The method as recited in claim 23, wherein the preamble
includes N number of sub-preambles generated by a signature code of
a set of signature codes used for the RACH to which the mobile
station tries to access, N being an integer larger than 2.
29. The method as recited in claim 28, wherein the preamble
includes: N-1 number of first sub-preambles having the same code;
and a second sub-preamble which is a conjugate code of the first
sub-preambles.
30. The method as recited in claim 28, wherein the preamble
includes: N-1 number of first sub-preambles having the same code;
and a second sub-preamble with an inversed sign of the first
sub-preambles.
31. The method as recited in claim 28, wherein the preamble is
composed of a continuum of the N number of sub-preambles.
32. The method as recited in claim 28, wherein the preamble is
composed of the N number of sub-preambles apart in a predetermined
interval intermittently.
33. The method as recited in claim 26, wherein the preamble and the
message are generated by a spreading code corresponding to the
selected sub-access frame in the RACH.
34. The method as recited in claim 24, wherein the access frame is
larger than a maximum difference of round trip delay times between
the different mobile stations and the satellite access network.
35. The method as recited in claim 27, wherein the access frame is
larger than a sum of a maximum difference of round trip delay times
between the different mobile stations and the satellite access
network, and the twice of a maximum transmission time offset.
36. The method as recited in claim 26, wherein a length of the
sub-access frame is the same as that of the radio frame broadcasted
from the satellite access network on the control channel.
37. The method as recited in claim 23, wherein the AI signal is any
one of: a positive AI signal informing of an acquisition indication
of the preamble or the message; and a negative AI signal informing
that the use of the RACH through which the preamble or the message
is transmitted is not permitted currently.
38. The method as recited in claim 23, wherein the step a) includes
the steps of: a1) selecting a RACH access service class of the
message to be transmitted; a2) selecting an access frame to be used
for transmission of the preamble and the message, a signature code
to be used for generating the preamble and a spreading code
corresponding to the RACH to be accessed to; and a3) transmitting
the preamble and the message generated by the signature code and
the spreading code successively based on the access frame as a time
unit.
39. The method as recited in claim 38, further including the step
of: a4) determining if the persistence test is satisfied at or
before the step a3).
40. The method as recited in claim 38, wherein if none of the
positive or negative AI signal is received from the satellite
access network at the step b), the step c) includes the steps of:
c1) performing the steps a2) and a3) repeatedly by increasing the
transmission power of the preamble and the message until any one of
the positive AI signal and the negative AI signal is received; and
c2) if the repetition number of the step c1) exceeds a
predetermined maximum number of power increase retransmissions,
performing the steps a1) to a3) repeatedly for the new
retransmission period.
41. The method as recited in claim 38, wherein if the AI signal of
the step b) is a positive AI signal, the step c) includes the step
of: waiting for reception of a message response transmitted from
the satellite access network.
42. The method as recited in claim 38, wherein if the AI signal of
the step b) is a negative AI signal, the step c) includes the step
of: performing the steps a1) to a3) repeatedly after a
predetermined time.
43. The method as recited in claim 38, wherein the step a2) further
includes the step of selecting any one of the plurality of
sub-access frames into which the access frame is divided; and
wherein the spreading code corresponds to the selected sub-access
frame in the RACH to be accessed to, and in the step a3), the
preamble and the message are transmitted based on the starting
point of the access frame or the sub-access frame.
44. The method as recited in claim 43, wherein the step a2) further
includes the step of selecting a transmission time offset; and
wherein in the step a3), the preamble and the message are
transmitted after the selected transmission time offset from the
starting point of the access frame or the sub-access frame.
45. The method as recited in claim 38, wherein the step a2)
includes the step of: updating the parameters related to the RACH
access based on the RACH information obtained from the control
channel broadcasted from the satellite access network.
46. The method as recited in claim 43, wherein in the step b), the
AI signals for the preamble and the message are received after a
predetermined preamble-AI time is passed from the starting point of
the access frame or the sub-access frame in which the preamble and
the message are transmitted in the step of a3).
47. The method as recited in claim 43, wherein in the step b), the
AI signals for the preamble and the message are received after a
predetermined preamble-preamble time is passed from the starting
point of the access frame or the sub-access frame in which the
preamble and the message are transmitted in the step of a3).
48. The method as recited in claim 47, wherein the preamble-AI time
is shorter than the preamble-preamble time by as much as one access
frame.
49. The method as recited in claim 46, wherein the preamble-AI time
is an integer multiple of the access frame.
50. The method as recited in claim 47, wherein the
preamble-preamble time is an integer multiple of the access
frame.
51. The method as recited in claim 23, wherein the AI signal for
the preamble or the message is transmitted based on the radio frame
broadcasted from the satellite access network on the control
channel.
52. The method as recited in claim 38, wherein the step a2), a
persistence test probability, an initial transmission power offset
and a transmission power increase value are further selected, and
wherein an available spreading code, a set of available signatures,
a persistence test probability, an initial transmission power
offset and the transmission power increase value are selected
differently according to the RACH access service class of the step
of a1).
53. The method as recited in claim 37 or 51, wherein in case that
the reception time of the AI signal is longer than that of the
radio frame, the step b) includes the steps of: b1) if at least one
of the plurality of AI values included in the AI signal is a
positive AI value, combining all the AI signals into a positive
value; and b2) if each of the AI values included in the AI signal
is either a negative AI value or a zero, combining all the AI
signals into a negative value.
54. A mobile station for accessing a random access channel (RACH)
through which a plurality of mobile stations transmit messages to a
satellite access network, the mobile station comprising: a
transmission resource determination means for selecting an access
frame for transmitting a message and a preamble having a plurality
of sub-preambles, a sub-access frame among the sub-access frames
into which the access frame is divided, a transmission time offset,
a signature code for generating the preamble and a spreading code
corresponding to the selected sub-access frame and the selected
RACH; a generation means for modulating the preamble and the
message suitable to be transmitted to the RACH by using the
signature code and the spreading code determined in the
transmission resource determination means; a transceiver means for
transmitting the preamble and the message successively after the
selected transmission time offset is passed from the starting point
of the access frame or the sub-access frame, and for receiving
acquisition indicator (AI) signals corresponding to the preamble
and the message; and a transmission determination means for
determining whether to retransmit the preamble and the message or
wait for a response to the message according to the received AI
signals.
55. A satellite access network for accessing a random access
channel (RACH) through which a plurality of mobile stations
transmit messages to the satellite access network, the satellite
access network comprising: a transceiver means for receiving a
preamble and a message that are transmitted successively from the
plurality of mobile stations; and an acquisition indicator (AI)
signal for generating an positive AI signal including the
acquisition indication information of the preamble and the message,
or a negative AI signal including information that the use of the
RACH through which the preamble and the message is not permitted
currently.
56. A random access channel (RACH) access apparatus of a mobile
station, for accessing a random access channel (RACH) through which
a plurality of mobile stations transmit messages to a satellite
access network, the apparatus comprising: a transmission resource
determination means for selecting an access frame for transmitting
a message and a preamble having a plurality of sub-preambles, a
sub-access frame among sub-access frames into which the access
frame is divided, a transmission time offset, a signature code for
generating the preamble and a spreading code corresponding to the
selected sub-access frame and the selected RACH; a generation means
for modulating the preamble and the message suitable to be
transmitted to the RACH by using the signature code and the
spreading code determined in the transmission resource
determination means; a transceiver means for transmitting the
preamble and the message successively after the selected
transmission time offset from the starting point of the access
frame or the sub-access frame, and for receiving acquisition
indicator (AI) signals for the transmitted preamble and the
message; and a transmission determination means for determining
whether to retransmit the preamble and the message or wait for a
response to the message according to the received AI signals.
57. A random access channel (RACH) access apparatus of a satellite
access network, for accessing a random access channel (RACH)
through which a plurality of mobile stations transmit messages to
the satellite access network, the apparatus comprising: transceiver
means for receiving a preamble and a message that are transmitted
successively from the plurality of mobile stations; and an
acquisition indicator (AI) signal for generating an positive AI
signal including acquisition indication information of the preamble
and the message, or a negative AI signal including information that
the use of the RACH through which the preamble and the message is
not permitted.
Description
The present patent application is a non-provisional application of
International Application No. PCT/KR/01/01746, filed Oct. 17,
2001.
TECHNICAL FIELD
The present invention relates to a random access channel (RACH)
access apparatus which is used when a mobile station having a
characteristic of bursty transmission needs to transmit a short
message without a prior radio link establishment in a code division
multiple access (CDMA) satellite mobile communication system, and
the method therefor.
BACKGROUND ART
A random access channel (RACH) is a channel used for transmitting a
short message over one or two frames in uplink. The channel
structure of the RACH and the RACH access process are disclosed in
the 25.211 and 25.214 of the Technical Specification (TS) of the
Third Generation Partnership Project (3GPP).
The RACH is an uplink transmission channel in which signals are
always received from the entire cell. The RACH features a collision
risk and an open loop power control.
Data packets of a medium length around 50 frames at largest are
transmitted through a common packet channel (CPCH), and data of
over 50 frames such as voice data are transmitted through a
dedicated channel.
In case of a terrestrial mobile communication system for
International Mobile Telecommunication-2000 (IMT-2000), a mobile
station transmits a preamble to an access network through a random
access channel (RACH) before sending out a message.
If any acquisition indicator signal of the transmitted preamble
does not arrive from the access network within a predetermined
time, the mobile station increases transmission power, retransmits
the preamble and waits for the acquisition indicator signal from
the access network again.
When the mobile station that has transmitted the preamble to the
access network receives the acquisition indicator signal from the
network within the predetermined period, it finally sends out the
message. In short, an acquisition indication procedure for the
preamble reception is performed before a message is
transmitted.
Also, it can be checked out by the mobile station whether or not
the transmitted message is received in the access network without
error after receiving a response for the message--not an
acquisition indicator signal of the message but a response signal
of the message content--from the access network. To transmit the
response to the message to the mobile station, it should be
processed in the upper layers of the access network and it takes
time to do it.
There are a couple of problems and requirements to apply the RACH
access method of the terrestrial mobile communication system to a
satellite mobile communication system.
First, since propagation delay time in the link between a mobile
station and an access network is generally less than 1 ms in the
terrestrial mobile communication system, the waiting time for an
acquisition indicator signal after transmitting a preamble is
short. Therefore, in the terrestrial mobile communication system, a
preamble is sent out prior to a message, and after acknowledging
the preamble, the message is transmitted. This way, the probability
for a successful message transmission can be heightened.
However, in a satellite mobile communication system, the
propagation delay time in the link between a mobile station and a
satellite access network is more than decades or hundreds of ms,
and naturally the waiting time for an acknowledge signal after the
transmission of a preamble is several times of the propagation
delay time. Accordingly, when the RACH access method of the
terrestrial mobile communication system is applied to the satellite
mobile communication system, it takes severely long time to
transmit a preamble and acquire the indication on the successful
reception of the preamble because of long propagation delay time.
As a result, there is a problem that the message transmission delay
becomes very large.
Secondly, the distance between a satellite or an earth station of
the satellite access network and a mobile station in the satellite
mobile communication system is much more distant than that between
a base station and a mobile station in the terrestrial mobile
communication system, i.e., the propagation delay is long, and thus
the received power of the preamble is relatively smaller.
Therefore, the probability of the successful preamble reception at
the satellite access network side is very low. In addition, in case
of using a low earth orbit satellite, the Doppler shift effect due
to satellite movement occurs and reaches as far as tens of kHz.
Therefore, in a satellite mobile communication system, there is a
problem that much energy should be assigned to the preamble in
order to enhance the reception probability in case of transmitting
a single preamble as in the terrestrial mobile communication
system.
Thirdly, in the terrestrial mobile communication system, the
problem of message transmission delay is not severe even when the
procedures of transmitting a preamble prior to a message,
confirming the acquisition of the preamble, transmitting a message
and receiving a response to the message--which is not an
acquisition indicator signal for whether the message is received,
but for the content of the message--are performed, because the link
delay time between the mobile station and the base station is
short.
Also, for the terrestrial mobile communication system, although the
preamble and the message are transmitted together in the
conventional ALOHA protocol and the response to the message is
received without an acquisition indicator (AI) signal for the
preamble acquisition, the message transmission delay time including
the time for processing a response to a message in the upper layers
in the access network does not cause any problem, thanks to short
propagation delay time between the mobile station and the base
station. In case of making access to the RACH in the ALOHA
protocol, the mobile station transmits a message together with its
preamble and knows whether the message and the preamble are
successfully received at the base station without error by
receiving a response to the transmitted message. Therefore, a
processing time in the upper layers of the access network is
required including the access network to process the response to
the message from the mobile station and transmit the response to
the mobile station. In short, only after the time for signaling and
processing in the upper layers, which are necessary inside the
network, passes, a mobile station can receive the response to the
message and confirm if the message is received without error.
Therefore, in the terrestrial mobile communication system, although
the message transmission delay includes the time for signaling and
processing a response to the message in the upper layers, the time
delayed until the mobile station receives the response to the
message does not become a big problem.
However, as described above, in the satellite mobile communication
system, the propagation delay time reaches tens or hundreds of ms,
and the waiting time for the response to a message after the
message transmission is several times of the propagation delay
time. Therefore, it takes seriously long time until the preamble is
acquired, the successful reception of the preamble is confirmed,
and the response to the message is received.
Further, although it may be different according to environments of
the mobile communication system, in general, the time for signaling
and processing a response to a message is larger than the link
propagation delay time. Accordingly, the link propagation delay,
which is negligible in the terrestrial mobile communication system,
is severe in the satellite mobile communication system.
Fourthly, in case of a mobile communication system using a slotted
RACH method, where a mobile station transmits a packet through the
RACH to be received in the access network within a slot, the mobile
station should carefully control the packet transmission time to
stay within the precision of the slot of the access network. To
synchronize the reception time of a packet with a slot at the
access network, the propagation delay time between the mobile
station and the access network should be figured out precisely and
the packet transmission time should be controlled. Therefore,
before the packet is transmitted through the RACH, a transmission
for slot synchronization and a feedback procedure for the
synchronization between the mobile station and the access network
should be performed, or the exact propagation delay time should be
figured out by using a signal from an external device such as a
global positioning system (GPS) and confirming the exact location
of the mobile station and the satellite.
For these reason, in a satellite mobile communication system, it is
preferred to simplify the synchronization of the RACH.
Fifthly, as mentioned above, since the received power of the
packets at the satellite access network is deteriorated, when two
mobile stations close to each other transmit packets at the same
time, the satellite access network receives the packets almost
simultaneously, such that the interference to each other is
increased and the packet reception probability is seriously
dropped. When packets transmitted from a plurality of mobile
stations simultaneously are received by a satellite access network,
the reception times of the packets at the satellite access network
are centralized into a particular time duration, which depends on
the difference of round trip delay times. Therefore, when the round
trip delay time difference is very small, the reception time of
packets from the mobile stations are centralized in a particular
time, such that the interference to each other is increased and the
packet reception probability is seriously dropped.
Finally, in case of the terrestrial mobile communication system,
after transmitting a message, the mobile station will waits for a
response to the message from the access network during a
predetermined time, and if the access network doesn't receive the
message successfully, there is a problem that it would take at
least two round trip delays from the transmission time of the
previous preamble for the mobile station to retransmit the
preamble. This problem turns out to be more serious in the
satellite mobile communication system, in which link delay time is
much longer than that of the terrestrial mobile communication
system.
DISCLOSURE OF INVENTION
It is, therefore, an object of the present invention to provide an
apparatus and method for accessing a random access channel (RACH)
to shorten transmission delay time in the RACH access process of a
satellite mobile communication system.
It is another object of the present invention to provide an
apparatus and method for accessing a random access channel (RACH)
to enhance the satellite access network's reception probability of
preambles and messages transmitted from the mobile stations
It is still another object of the present invention to provide an
apparatus and method for accessing a random access channel (RACH)
to reduce the transmission power of the transmitted packets, while
enhancing the reception probability of packets transmitted from
mobile stations.
It is still another object of the present invention to provide an
apparatus and method for accessing a random access channel (RACH)
to shorten waiting time of the mobile station for the
acknowledgement to the message that a mobile station has
transmitted, by ruling out the time for signaling and processing a
response to a message in the upper layers in the RACH access
process of the satellite mobile communication system
It is still another object of the present invention to provide an
apparatus and method for accessing a random access channel (RACH)
to simplify the synchronization needed for packet transmission in
the satellite mobile system.
It is still another object of the present invention to provide an
apparatus and method for accessing a random access channel (RACH)
to reduce interference between packets as well by decentralizing
the packet reception time on a frame basis, when the satellite
access network receives packets from a plurality of mobile
stations.
It is still another object of the present invention to provide an
apparatus and method for accessing a random access channel (RACH)
to shorten the waiting time of a mobile station for message
retransmission by making the mobile station receive an acknowledge
signal of message acquisition from the physical layer of the
satellite access network, instead of a response to the message,
when the satellite access network successfully receives a preamble
but fails to receive the message.
Those skilled in the art will be able to easily figure out another
objects and advantages of the present invention from the drawings,
detailed description of the invention and claims of this
specification.
In accordance with one aspect of the present invention, there is
provided a method for accessing a random access channel (RACH)
through a plurality of mobile stations transmit messages to a
satellite access network, including the steps of: a) receiving a
preamble and the message transmitted successively from the mobile
station; and b) transmitting an acquisition indicator (AI) signal
for the preamble and the message to the mobile station.
In accordance with another aspect of the present invention, there
is provided a method for accessing a random access channel (RACH)
through which a plurality of mobile stations transmit messages to a
satellite access network, including the steps of: a) transmitting a
preamble and the message successively to the satellite access
network; b) receiving an acquisition indicator (AI) signal for the
preamble or the message from the satellite access network; and c)
retransmitting the preamble or the message based on the AI signal,
or waiting for the response for the message.
In accordance with further another aspect of the present invention,
there is provided a mobile station for accessing a random access
channel (RACH) through which a plurality of mobile stations
transmit messages to a satellite access network, the mobile station
including: a transmission resource determination unit for selecting
an access frame for transmitting a preamble having a plurality of
sub-preambles, a sub-access frame among the sub-access frames into
which the access frame is divided, a transmission time offset, a
signature code for generating the preamble and a spreading code
corresponding to the selected sub-access frame; a generation unit
for modulating the preamble and the message suitable to be
transmitted to the RACH by using the signature code and the
spreading code determined in the transmission resource
determination unit; a transceiver unit for transmitting the
preamble and the message successively after the selected
transmission time offset is passed from the starting point of the
access frame or the sub-access frame, and for receiving acquisition
indicator (AI) signals corresponding to the preamble and the
message; and a transmission determination unit for determining
whether to retransmit the preamble and the message or wait for a
response to the message according to the received AI signals.
In accordance with still further another aspect of the present
invention, there is provided a satellite access network for
accessing a random access channel (RACH) through which a plurality
of mobile stations transmit messages to the satellite access
network, the satellite access network including: a transceiver unit
for receiving a preamble and the message that are transmitted
successively from the plurality of mobile stations; and an
acquisition indicator (AI) signal for generating an positive AI
signal including the acquisition indication information of the
preamble and the message, or a negative AI signal including
information that the use of the RACH through which the preamble and
the message is not permitted.
In accordance with still further another aspect of the present
invention, there is provided a random access channel (RACH) access
apparatus of a mobile station, for accessing a random access
channel (RACH) through which a plurality of mobile stations
transmit messages to a satellite access network, the apparatus
including: a transmission resource determination unit for selecting
an access frame to be used for transmitting a preamble having a
plurality of sub-preambles and a message, a sub-access frame among
sub-access frames into which the access frame is divided, a
transmission time offset, a signature code for generating the
preamble and a spreading code corresponding to the selected
sub-access frame and the selected RACH; a generation unit for
modulating the preamble and the message suitable to be transmitted
to the RACH by using the signature code and the spreading code
determined in the transmission resource determination unit; a
transceiver unit for transmitting the preamble and the message
successively after the selected transmission time offset from the
starting point of the access frame or the sub-access frame, and for
receiving acquisition indicator (AI) signals for the transmitted
preamble and the message; and a transmission determination unit for
determining whether to retransmit the preamble and the message or
wait for a response to the message according to the received AI
signals.
In accordance with yet further another aspect of the present
invention, there is provided a random access channel (RACH) access
apparatus of a satellite access network, for accessing a random
access channel (RACH) through which a plurality of mobile stations
transmit messages to the satellite access network, the apparatus
including: transceiver unit for receiving a preamble and the
message that are transmitted together from the plurality of mobile
stations successively; and an acquisition indicator (AI) signal for
generating an positive AI signal including acquisition indication
information of the preamble and the message, or a negative AI
signal including information that the use of the RACH through which
the preamble and the message is not permitted.
According to the present invention, a preamble and a message for
the access to an RACH are transmitted successively to shorten the
time delayed for packet transmission on the satellite link between
the mobile station and the satellite access network.
Also, an acknowledgement signaling on the acquisition of preamble
or message are processed in the physical layer of an earth station
or a satellite of a satellite access network to reduce an
acknowledgement waiting delay time of a mobile station.
Also, a preamble is composed of a plurality of sub-preambles to be
repeated successively and transmitted to the satellite access
network to enhance the reception probability of the preamble and
the message in the poor reception power environment of the
satellite link. At this point, the code of the last sub-preamble
may be an inversed code of the preceding sub-preamble code or a
conjugate code so as to distinguish the preamble from the message
coming right afterwards.
Further, the repeated sub-preambles can be transmitted
intermittently to heighten the power efficiency on a fading
channel.
Also, to simplify the synchronization of the reception time of
packet which a mobile station transmits in the satellite mobile
communication system, an access frame that will become a time unit
for the transmission of the preamble and the message is
synchronized with the reception time of downlink control channel
frame transmitted from the satellite access network towards the
mobile station. Here, the length of the access frame may be set up
larger than the maximum round trip delay time, and the time unit
for the mobile station receiving an AI signal and for
retransmitting the preamble and the message can be set up based on
the access frame.
Also, in order to decentralize the reception time of preambles and
messages transmitted from the mobile stations, each of the
plurality of mobile stations can differentiate the packet
transmission time by dividing an access frame into a number of
sub-access frames.
Also, in order to decentralize the reception time of preambles and
messages transmitted from the mobile stations, each of the
plurality of mobile stations can differentiate the packet
transmission time by setting the transmission time point of the
preamble and message with a transmission time offset, which each of
the multiple numbers of mobile stations selects independently, from
the starting point of the access frame or the sub-access frame.
Also, a preamble AI signal can be used as a message AI signal as
well to reduce the waiting time of the mobile station for an
acknowledgement to the message that a mobile station has
transmitted.
Also, a priority may be given according to the kind of random
access message by assigning the set of an access frames available
in an RACH, spreading code, probability in a persistence
examination, initial message transmission power and the size of
power increase step in retransmission differently, according to the
kind of message.
BRIEF DESCRIPTION OF DRAWINGS
The above and other objects and features of the present invention
will become apparent from the following description of the
preferred embodiments given in conjunction with the accompanying
drawings, in which:
FIG. 1 is a schematic diagram of a satellite access network and a
mobile station describing a satellite mobile communication
environment to which the present invention is applied;
FIG. 2 is a block diagram showing a functional structure of the
mobile station of FIG. 1;
FIG. 3 is a block diagram illustrating a functional structure of a
random access channel (RACH) access apparatus adopted in an earth
station or a satellite of FIG. 1;
FIG. 4 is a timing diagram of a frame and a packet describing an
RACH access method in accordance with an embodiment of the present
invention;
FIG. 5 is a structural diagram of a preamble and a message
describing the RACH access method in accordance with an embodiment
of the present invention;
FIG. 6 is a structural diagram of a preamble describing an
intermittent transmission of a sub-preamble in accordance with an
embodiment of the present invention;
FIG. 7 is a diagram describing a power increase retransmission and
a retransmission cycle in the random access channel (RACH) in
accordance with an embodiment of the present invention; and
FIG. 8 is a flow chart showing the RACH access method in accordance
with an embodiment of the present invention.
MODES FOR CARRYING OUT THE INVENTION
Other objects and aspects of the invention will become apparent
from the following description of the embodiments with reference to
the accompanying drawings, which is set forth hereinafter. First of
all, attention should be paid to a point that for the same
constituent, the same reference numeral has been given, although it
is shown in different drawings. Also, descriptions thought to
unnecessarily interrupt a correct understanding on this invention
is pulled out.
FIG. 1 represents a schematic diagram of a satellite access network
and a mobile station for describing a satellite mobile
communication environment to which the present invention is
applied.
Referring to the drawing, a satellite mobile communication
environment to which the present invention is applied includes: a
mobile station 170 for generating a message and transmitting a
preamble and the message through a random access channel (RACH);
and a satellite access network 100 for transmitting an acquisition
indicator (AI) signal to the preamble and message transmitted from
the mobile station 170 through a acquisition indicator channel and
relaying the message to an terrestrial core network (not shown in
this drawing). The satellite access network 100 includes: a control
station 150 for controlling the satellite access network 100 and
cooperating with the terrestrial core network; a satellite 110; and
an earth station 130, thereby providing a connection between the
mobile station 170 and the terrestrial core network.
FIG. 2 is a block diagram showing a functional structure of the
mobile station of FIG. 1. As illustrated in the drawing, the mobile
station 170 of the present invention includes a mobile station
transceiver unit 209, which transmits preambles and messages to the
satellite access network 100 successively during an access frame
and receives a preamble AI signal, a message AI signal or a
response to a message from the satellite access network 100. The
mobile station transceiver unit 209 can transmit a preamble and
message with a predetermined transmission offset time difference
(T.sub.off) from a starting point of an access frame or a
sub-access frame, as will be described later.
Also, the mobile station 170 includes a message processing unit 201
for converting user data into message so as to transmit them
through the RACH.
Also, the mobile station 170 of the present invention further
includes a transmission resource determination unit 203 for
selecting a parameter needed to access the RACH, such as a
signature code for generating preambles and an access frame for
transmitting the preambles and the messages. The transmission
resource determination units 203 of all mobile stations trying to
access the RACH in the present invention selects the access frame
and the sub-access frame independently, and the access frame is set
larger than the maximum difference of the round trip delay time
between the mobile station 170 and the satellite access network
100.
Also, the mobile station 170 generates a preamble by using a
signature code in accordance with the present invention, and
includes a transmission unit 205 for receiving message from the
message processing unit 201 and demodulating them into the signal
suitable for the RACH. Accordingly, the transmission unit 205
includes a preamble generator and a packet spreader.
Finally, the mobile station 170 includes a transmission
determination unit 207 for determining whether a preamble and a
message should be retransmitted based on an acquisition indicator
signal received by the mobile station transceiver unit 209 from the
satellite access network 100. When an acquisition indicator signal
is not received from the satellite access network 100, the
transmission determination unit 207 activates the transmission
resource determination unit 203 and the transmission unit 205 to
increase the power and retransmit the preamble or the message until
the acquisition indicator signal is received or the number of
retransmissions reaches a predetermined maximum value. The
transmission determination unit 207 updates parameters related to
the RACH based on the channel information acquired from the
satellite access network 100 through the control channel 401 (refer
to FIG. 4).
FIG. 3 is a block diagram illustrating a functional structure of a
random access channel (RACH) access apparatus adopted in an earth
station or a satellite of FIG. 1.
A random access channel (RACH) access apparatus for the satellite
access network 100 of the present invention can be embodied in the
satellite 110 or the earth station 130. As shown in the drawing,
the RACH access apparatus includes a satellite system transceiver
unit 301 for receiving preambles and messages transmitted from a
plurality of mobile stations and transmitting AI signals and
responses of the messages to the mobile stations; and a message
transceiver unit 303 for relaying message to the earth network.
Meanwhile, the RACH access apparatus further includes an
acquisition indicator (AI) signal generation unit 305 for
generating AI signals to the preambles and messages received from
the satellite system transceiver unit 301 in accordance with the
present invention.
In this invention, in the reverse uplink from the mobile station
170 to the satellite access network 100, there can be one or more
than one random access channels (RACHs) 405 and 409. In the forward
downlink from the satellite access network 100 to the mobile
station 170, there is a control channel 401 and one or more than
one acquisition indicator channels 403 and 407.
The satellite access network 100 broadcasts system information and
parameters for other channels as well as the RACH, through the
control channel 401 (refer to FIG. 4) to the mobile station 170.
The mobile station 170 receives parameters related to the access to
the RACH 405 or 409, and transmits a preamble for the access to the
RACH 407 and a message 425 to the satellite access network 100,
using the parameters transmitted through the downlink control
channel 401.
The satellite access network 100 that has received the preamble 415
and the message 425 transmits AI signals 423 and 433 of the
preamble and the message to the mobile station 170 through the
acquisition indicator channels 403 and 407.
FIG. 4 is a timing diagram of a frame and a packet describing the
RACH access method in accordance with an embodiment of the present
invention, and FIG. 5 is a structural diagram of a preamble and a
message describing the RACH access method in accordance with an
embodiment of the present invention. FIG. 6 is a structural diagram
of a preamble describing an intermittent transmission of a
sub-preamble in accordance with an embodiment of the present
invention.
Referring to FIG. 4, the mobile station 170 within the service
coverage of the satellite 110 synchronizes the time points of the
access frame 421, which is a time interval of the access channel,
with the sub-access frames 431, 441 based on the reception time of
the downlink radio frame 411, a time interval of a control channel
401 transmitted from or through the satellite 110.
In the satellite mobile communication system employing an
asynchronous CDMA method, the transmission time point of each
mobile station should be exactly controlled so that the earth
station (just in case a satellite simply plays the role of
amplifying and relaying the signal) or the satellite (in case a
satellite plays the role of the transceiver of the satellite access
network) can receive packets transmitted from the mobile stations
through the RACH within the precise of a slot. This slot-based RACH
access method should include the procedures for measuring the link
path delay and controlling the transmission time. To simplify the
additional procedure of controlling the reception time point in the
satellite or the earth station, that is required in the
conventional slot-based RACH access method, each mobile station 170
sets its transmission time point of a preamble 415 and a message
425 based on the radio frame 411 received from the satellite access
network 100, instead of controlling the reception time point at the
satellite or the earth station into a certain slot time.
In the present invention, the access frame instead of the slot is
used for a time unit of the RACH transmission. The access frame
includes multiple slots and has a longer length than the slot. The
length of the access frame 421 should be set larger than the
maximum difference of the round trip delays of two mobile stations
170 located in different places of a spot beam coverage of the
satellite 110.
The distances between the satellite 110 and the mobile stations 170
at different locations can be different respectively. Accordingly,
preambles 415 and messages 425 transmitted by the mobile stations
at different locations are received in the satellite access network
100 with a time difference as much as the round trip delay
difference between the mobile stations 170.
Further, when receiving AI signals of the preamble 415 and the
message 425 from the satellite access network 100, each of the
mobile stations 170 receives the AI signal with a time difference
as much as the round trip delay difference between the mobile
stations 170.
Therefore, in case of controlling the packet transmission time of
the mobile station 170 into the time unit of the access frame 421
in accordance with the present invention, the length of the access
frame 421 can be set larger than the maximum difference of the
round trip delay time in a satellite spot beam.
In short, the length of the access frame 421 can be set larger than
the maximum difference of the round trip delay time so as to be
able to determine whether the packets the satellite access network
100 receives are transmitted based on the same access frame 421, or
they are transmitted based on different access frames 421.
The satellite access network 100 determines the time point of the
access frame 421 to be used for transmitting AI signals 423 or 433,
based on the access frame 421 in which the satellite access network
100 received the packet. Each mobile station 170, also, determines
the time point of the access frame in which the AI signal 423 or
433 to the preamble and message it has transmitted will be
transmitted by the satellite access network 100, according to the
time point of the access frame 421 in which it has transmitted.
At this point, FIG. 4 is illustrating an embodiment in which the
RACH 409 is composed of access frames twice as long as the radio
frame 411. That is, the drawing is showing a case where the length
of the access frame 421 is twice the length of a radio frame 411.
This case means that the maximum difference of the round trip delay
in a spot beam of the satellite 100 is longer than the time length
of the radio frame 411 and shorter than that of two radio frames
411.
The length of the access frame 421 can be set in integer multiples
of the radio frame 411 for the sake of convenience in control. That
is, it is set in a length n time(s) the radio frame 411, n being an
integer that satisfies the following Equation (1).
(n-1).times.T.sub.f<.DELTA.D.sub.max+T.sub.proc<n.times.T.sub.f
Eq. (1)
In Equation (1), T.sub.f, .DELTA.D.sub.max and T.sub.proc denote
the duration of a radio frame (i.e., radio frame length), the
maximum difference time of round trip delay in a spot beam and the
processing time required for the reception and transmission of
related signals, respectively.
However, it is obvious to those skilled in the art of the present
invention that the determination of a radio frame 411 and an access
frame 421 described in the drawing can be varied according to the
satellite mobile communication environments and system designs.
Accordingly, the present invention is not limited to the method of
determining the radio frame 411 and the access frame 421 as
described in the drawing.
If the maximum difference of round trip delay is smaller than the
radio frame 411, the packet transmission time unit of an RACH can
be set in a radio frame 411 instead of an access frame 421.
As described above, the access frame 421 is longer than the radio
frame 411, the access frame 421 can be divided into several
sub-access frames 431, 441. In case that the access frame 421 is an
n (n being an integer) multiple of the radio frame 411, n number of
sub-access frames can be set in a single access frame 421.
A mobile station 170 selects an access frame 421 and one of the
sub-access frames 431 and 441 within the access frame 421, and
transmits the preamble 415 and the message 425 based on the
selected sub-access frame 431 or 441. In this case, a spreading
code (S.sub.pre, i) corresponding to the sub-access frame 431 or
441 of the RACH 409 can be set differently. In other words, the
spreading code corresponding to the sub-access frame 431 of the
RACH 409 selected by the mobile station 170 is different from that
corresponding to the sub-access frame 441. Accordingly, it becomes
possible for the satellite access network 100 to distinct preambles
415 and messages 425 transmitted at the different sub-access frames
431 and 441 thanks to different spreading codes.
Further, the transmission time point of a preamble 415 and a
message 425 can be set to be offset as much as the transmission
offset time (T.sub.off [chip]) from the starting point of the
sub-access frames 431, 441 the mobile station has selected. The
transmission offset time (T.sub.off) is a value the mobile station
170 randomly selects from -T.sub.off,max [chip] determined by a
predetermined maximum transmission offset time to T.sub.off,max
[chip].
As shown above, the preamble 415 and message 425 are transmitted
based on the sub-access frame 431, 441 and the transmission offset
time (T.sub.off). The reason is that if mobile stations 170 located
closed to each other transmit preambles 415 and messages 425 at a
time point of the same access frame 421, the satellite access
network 100 would receive the preambles 415 and messages 425 almost
at the same time point, thereby generating interference between
packets.
When preambles 415 and messages 425 are transmitted from a
plurality of mobile stations 170 based on the starting point of the
same access frame 421 and received by the satellite access network
100, they can be centralized into a particular time duration. The
centralized time duration depends on the difference of a round trip
delay time. Accordingly, in case that the round trip delay time
difference is very shorter than the time length of the access frame
421, the preambles 415 and messages 425 are centralized into a
corresponding time duration and a serious interference can be
caused.
Therefore, according to the present invention, the interference can
be prevented by decentralizing the transmission time points of the
preambles 415 and messages 425 by the transmission offset time
(T.sub.off) and the sub-access frames 431, 441 selected by each of
the mobile stations 170.
Referring FIG. 4, the RACH 405 is illustrating a case where a
mobile station transmits a preamble 415 and a message 425 without
consideration of the access frame 421, the sub-access frame 431 or
441 and the access offset time (T.sub.off) On the other hand, the
RACH 409 is showing a case where a preamble 415 and a message 425
are transmitted in consideration of the access frame 421, the
sub-access frame 431 or 441 and the access offset time
(T.sub.off).
Meanwhile, in case that the transmission offset time (T.sub.off) is
used as a standard of a packet transmission time point in
accordance with the present invention, Equation (1) is modified
into the following Equation (2), in which the maximum initial
transmission offset time is considered, because there exists time
gap between the -T.sub.off,max to T.sub.off,max as well as the
maximum difference of a round trip delay in the reception time
point at the satellite access network 100.
(n-1).times.T.sub.f<.DELTA.D.sub.max+T.sub.proc+2.times.T.sub.off,max)-
<n.times.T.sub.f Eq. (2)
Meantime, referring to FIGS. 4 and 5, a data packet transmitted
from a mobile station 170 to a satellite access network 100 through
an RACH 405 or 409 consists of a preamble 415 that is inserted to
make message reception 15 easy and a message 425 that contains the
actual information to transmit. The message 425 is what data is
spreaded by a spreading code (S.sub.pre,i) that corresponds to the
sub-access frame 431 or 441 which the mobile station 170 has
selected in the RACH 405 or 409 to transmit.
Referring to FIG. 5, the preamble 415 is composed of N.sub.p number
of sub-preambles 505, 515, each sub-preamble 505 or 515 being as
long as an L.sub.SP chip. Each sub-preamble 505 or 515 is expressed
as Equation (3) by a spreading code S.sub.pre,i of a corresponding
RACH 405 or 409 i, and a signature code C.sub.s for distinguishing
preambles transmitted from different mobile stations 170. Here, the
spreading code is composed of L.sub.sp chips which is the same as
the length of the sub-preamble 505 or 515. The length of the
spreading code is an integer multiple of the length of the
signature code, L.sub.sig chips, so that the signature code could
repeat in the sub-preambles 505, 515. Accordingly, in case that a
signature repeats N.sub.sig time(s) in the length of each
sub-preamble, the relation can be expressed as
L.sub.SP=N.sub.sig.times.L.sub.sig.
C.sub.pre(k)=S.sub.pre,i(k)*C.sub.s(k mod L.sub.sig), k=0,1,2, . .
. , L.sub.SP-1 Eq. (3)
In Equation (3), C.sub.pre denotes a sub-preamble code generated by
the s.sup.th signature code Cs and a spreading code S.sub.pre,i
corresponding to a sub-access frame i 431 or 441 that the mobile
station 170 has selected in the RACH 405 or 409. Each chip of a
code has a value of 1 or -1.
Also, k mod L.sub.sig denotes a remainder obtained when k is
divided by L.sub.sig.
The signature code C.sub.s is a sequence composed of a plurality of
symbols modulating spreading codes, which is used for the preambles
of the RACHs 405, 409. When using the signature, the satellite
access network 100 can acquire the preambles transmitted from a
plurality of mobile stations with different signatures if different
signatures are orthogonal to each other (therefore, for preambles,
too). Generally, a signature is composed of sequences each of which
is independent from each other, and for the signatures hadamard
sequences can be used.
To enhance the probability of acquiring preambles 415 in the
satellite access network 100, a sub-preamble is repeated N.sub.p
time(s), and the first N.sub.p-1 sub-preambles 505 are all composed
of the same code C.sub.pre while the last sub-preamble 515 includes
an inversed code -C.sub.pre or a conjugate code C.sub.pre*.
The sub-preamble C.sub.pre* 515 containing a conjugate code can be
embodied by the conjugate code S.sub.pre,i* of the spreading code
S.sub.pre,i that is used in the previous preamble C.sub.pre 505 and
corresponds to a sub-access frame 431 or 441 selected by the mobile
station 170 in the RACH 405 or 409.
An available signature code is defined in advance according to the
RACH 405 or 409 the mobile station is accessing to, and the mobile
station 170 selects one signature code among the signature codes
that correspond to the RACH 405 or 409.
For example, 16 signature codes may be set corresponding to 16
RACHs. Also, it is possible to set four signature groups. Each of
four signature groups includes four signature codes, and is mapped
to one of four RACHs. Here, the signature groups may or may not be
composed of the same signature codes. The mapping relation of the
RACH and the signature code is up to the choice of a system
designer.
One RACH 405 or 409 of RACH groups is selected in the upper layer
according to the class of the message 425 the mobile station 170 is
transmitting. The RACH 405 or 409 can be selected differently
according to the class of the message. Also, several classes may
use the same RACH.
However, it is obvious to those skilled in the art of the present
invention that the number of times of sub-preamble 415 repetition,
the distinction/selection of classes and the setup of a signature
code as illustrated in the drawing can be changed variously
according to the satellite mobile communication environment and the
system designer. Therefore, this invention should be understood not
limited to the repetition number of a sub-preamble 415, the
distinction/selection of classes and the setup of a signature code
as described in the drawing.
The probability of acquiring a preamble 415 in the satellite access
network 100 is increased as the sub-preamble repetition number
N.sub.p of the sub-preamble 505 in the preamble 415 increases. The
last sub-preamble 515, an inversed sub-preamble -C.sub.pre or a
conjugate preamble C.sub.pre*, informs that the next data is a
message 425. Therefore, although the first sub-preamble 505 of the
preamble 415 is not acquired at the time of initial reception, it
can be still acquired in the next coming sub-preambles 505. The
acquisition of preamble can be started at any sub-preamble 505 in
the continuum of the sub-preambles 505, and the terminating point
of the preamble 415 and the starting point of the message 425 can
be known by the last sub-preamble 515.
When the energy required for a successful transmission of a
preamble is said to be E, the whole energy E should be assigned to
a single preamble when the sub-preamble is not repeated.
However, when a preamble 415 is divided into N.sub.p number of
sub-preambles 505, 515 as in the present invention, each of the
sub-preambles should be assigned with an energy of E/N.sub.p.
Accordingly, when the instantaneous interference from other RACH
transmissions and other channels can be reduced, and the signal to
interference ratio of the whole preamble can be enhanced even if
the same energy is used. Therefore, the capability for acquiring a
preamble 415 can be enhanced.
Meanwhile, in general, signals received in the satellite mobile
communication environment go through fading, in which power changes
as time passes by. In the fading environment, instead of
transmitting the sub-preambles 505, 515 continuously as this
invention previously instructs, the mobile station can transmit a
single sub-preamble 505 or 515, which contains E/N.sub.p, during
the N.sub.g number of sub-preambles 505, 515 time durations
according to a predefined period N.sub.g (N.sub.g<N.sub.p) as
shown in FIG. 6, another embodiment of the present invention. The
transmission power required for the transmission of a preamble can
be reduced as shown in Equation (4).
E-N.sub.g.times.E/N.sub.p=E(1-N.sub.g/N.sub.p) Eq. (4)
The scale of fading changes according to the time, and the changing
speed is in proportion to the moving speed of the mobile station
170. Sub-preambles 505, 515 close to each other experience similar
scale of fading due to the time correlation of fading, while those
apart from each other show no similarity in scale. For example,
although the fading scale of the sub-preambles 505 positioned in
the front is large, that of those 505, 515 at the back may be
small. As mentioned above, the sub-preambles located close to each
other have a similar fading scale, while the sub-preambles located
remotely have an independent fading scale. Therefore, a diversity
effect of fading can be obtained as well as increasing the
efficiency of the transmission power by transmitting a sub-preamble
505 repeatedly with a predetermined time interval (N.sub.g),
instead of transmitting sub-preambles 505, 515 successively.
Referring to FIG. 6, sub-preambles 505, 515 are transmitted with an
interval of one (N.sub.g=1, see 601), two (N.sub.g=2, see 603) or
three (N.sub.g=3, see 605) sub-preamble 505, 515 time duration
during six (N.sub.P=6) identical sub-preambles 505, 515 time
duration. The last sub-preamble 515 has an inversed sign of the
preceding sub-preamble 505 code or a conjugate code of it as
described above.
Meanwhile, for the conventional random access method, it takes as
much a time as obtained in Equation (5) to transmit a preamble 415
and a message 425 and receive a response of the message from the
control station in the satellite mobile communication environment
of FIG. 1. T.sub.i=2t.sub.UL+2t.sub.FL+2t.sub.x+2t.sub.L+t.sub.RNC
Eq. (5)
where t.sub.UL denotes a propagation delay time over the link
between the mobile station 170 and the satellite 110;
t.sub.FL, a propagation delay time over the link between the
satellite 110 and the earth station 130;
t.sub.x, a time taken for processing packet reception and
transmission in the physical layer of the earth station 130;
t.sub.L, a propagation delay time between the earth station 130 and
the control station 150; and
t.sub.RNC, a time for the reception and transmission of a message
425 and for processing a response to the message 425 in the control
station 150. Therefore, t.sub.RNC includes the time for processing
the response to the message 425 in the upper layers of the control
station 150.
The transmission and reception of packet signals are processed in
the physical layer. The AI signals 423 and 433 are processed in the
physical layer, and the response to the message is processed in the
upper layers.
Accordingly, in the conventional method, even though the mobile
station has transmitted a message 425, if the message 425 is not
received in the earth station successfully and the response to the
message 425 is not transmitted to the mobile station from the
control station for a predetermined waiting time T.sub.0, the
mobile station attempts to retransmit the preamble 415 and the
message 425. So the waiting time T.sub.0 should be larger than
T.sub.1 at least.
However, in this invention where the preamble 415 and message 425
are transmitted successively, the retransmission delay time can be
saved, as the physical layer acknowledges the reception of the
preamble and the message transmitted from the mobile station as
soon as the preamble 415 and the message 425 are acquired before
the response to the message 425 is received in the upper layer of
the control station 150.
In general, a physical layer is embodied in the earth station 130.
Acquiring the preamble 415 transmitted from the mobile station 170,
the earth station 130 transmits an AI value (AI.sub.s)(see Equation
(8)), which corresponds to the acquired preamble 415, to the mobile
station 170.
On the other hand, the mobile station 170 attempts retransmission
instantly when the AI signal for the transmitted preamble 415 is
not received. Here, the time taken until the mobile station 170
performs retransmission can be expressed as Equation (6), and
compared to the case of Equation (5), the retransmission delay time
can be saved more. T.sub.2=2t.sub.UL+2t.sub.FL+2t.sub.x Eq. (6)
Further, in case that the function of the physical layer is
embodied in the satellite 110, the time taken until the mobile
station 170 retransmits the preamble 415 and the message 425 can be
reduced remarkably as shown in Equation (7).
T.sub.3=2t.sub.UL+2t.sub.x Eq. (7)
In the above Equations (6) and (7), t.sub.x denotes a time needed
for the transmission and reception of a signal in the physical
layer in accordance with the embodiment of the present invention.
As described above, if the AI signals 423, 433 are used to check
whether the preamble 415 is acquired successfully, it means the
time consumed to receive the preamble 415, generate AI signals 423,
433 when the preamble 415 is successfully received.
As another embodiment of the present invention, instead of being
used to see if the preamble 415 transmitted from the mobile station
170 is received successfully, the AI signals 423, 433 can be used
to check for both preamble 415 and message, whether they are
received without error.
In the satellite access network 100, if the preamble 415 is not
acquired successfully, the message 425 cannot be acquired, either.
Therefore, in this embodiment of the present invention, the
acquisition of the message 425 means that the preamble 415 also has
been acquired successfully.
Therefore, without modification of the AI signal 423 or 433 which
has been used for the purpose of checking if a preamble 415 has
been received successfully, this invention makes it possible to
inform the successful reception of a message 425 to the mobile
station 170 quickly. The successful reception of the message 425
can be confirmed in the satellite access network 100 using a cyclic
redundancy check (CRC) code, which is included in the message
425.
Consequently, in case of a success in the reception of a preamble
415 but failure in that of a message 425, the waiting time of the
mobile station 170 for retransmission can be reduced.
In Equations (6) and (7), t.sub.x denotes a time for the
transmission and reception of a signal in the physical layer in
accordance with an embodiment of the present invention. As
described above, in case that the AI signal 423 or 433 is used to
check if the message 425 is acquired successfully, it means a time
consumed to receive the preamble 415 and the ensuing message 425,
and generate the AI signal 423 or 433 if the message data 425 has
been received without error.
When the satellite access network 100 receives the preamble 415 and
the message 425 transmitted from the mobile station 170 in the
physical layer through the RACH 405 or 409, it transmits the
acquisition indicator (AI) signals 423, 433 to the mobile station
170 through the acquisition indicator channels 403, 407.
As an embodiment of the AI signals 423, 433 transmitted from the
satellite access network 100, the AI signals 423, 433 for the
preamble 415 are expressed as Equation (8).
.function..times..times..times.'.function..times..times..times..times.
##EQU00001##
where C.sub.s' denotes a code corresponding to the signature code
C.sub.s, which is used in the preamble 415 transmitted from a
mobile station 170, and N.sub.s is the total number of signatures.
Being an acquisition indication value, AI.sub.s hold the value 1,
-1 or 0 according to whether the preamble corresponding to the
signature code C.sub.s is successfully received. L.sub.AI is the
length of the AI signal 423 or 433.
Accordingly, the AI signal 423 or 433 has a value of C.sub.s'
(positive acquisition indication), -C.sub.s' (negative acquisition
indication) or 0 (no acquisition indication) according to the
acquisition indication (AI) value.
Likewise, in case of the AI signal 423 or 433 for the message 425
of the first embodiment of the present invention described before,
the acquisition indication value AI.sub.s has the value 1, -1 or 0
according to the reception of the message 425.
Therefore, the AI value AI.sub.s of the acquired preamble 415 or
message 425 has the value of 1, while that of non-acquired preamble
415 or message 425 have the value of 0. The AI value AI.sub.s of 0
means that the power of the AI signal 423 for the preamble 415 or
the message 425 of the signature code C.sub.s is 0. That is, the AI
signal corresponding to the signature code C.sub.s for the preamble
415 or the message 425 of the signature code C.sub.s is not
transmitted.
When a RACH is busy, for example, when the preamble 415 is received
successfully but: the receivers for the message 425 are lack, or
the system is overloaded, etc., all of the AI values or a
particular set of AI values (AI.sub.s) corresponding to the
preamble 415 or the message 425 can be assigned with the value of
-1 for system stabilization.
However, as shown in Equation (8), an AI signal with a particular
structure can be changed variously according to the satellite
mobile communication environment and system designer, which is well
known to those skilled in the art of this invention.
Therefore, the present invention is not limited to the AI signal of
a particular structure shown in Equation (8). It should be
understood that one of the positive acquisition indication, which
means a successful acquisition, no acquisition indication, which
means a failure in packet acquisition, and the negative acquisition
indication, which means the system is overload or lack of receivers
for the reception of message, can be transmitted to the mobile
station 170.
Meanwhile, as will be described later on, the sub-access frame 431
or 441 of the AI signal 423 or 433 that the mobile station 170
receives through the acquisition indicator channel corresponds to
the sub-access frame 431 or 441 the mobile station 170 has selected
for an initial transmission, the transmission of the preamble and
the message. That is, it is the sub-access frame 431 or 441 in the
same position. For instance, if the mobile station 170 selected a
first sub-access frame 431 for initial transmission, the mobile
station 170 waits until it receives acquisition indicator (AI)
signal 423 or 433 in the first sub-access frame 431, which is after
the preamble-AI time (T.sub.p-a) (see 407) from the starting point
of the sub-access frame 431.
In the acquisition indicator channel 403 of FIG. 4, one AI value
(AI.sub.s) corresponding to the preamble 415 and the message 425
transmitted through the RACH 405 or 409 is illustrated to be
included in the corresponding sub-access frame 431 or .441.
However, all the AI values (AI.sub.s) are included in the
sub-access frame 431 or 441 corresponding to the preambles 415 and
messages 425 transmitted through the same sub-access frame 431 or
441 of the RACH 405 or 409, which will be further described
later.
Upon acquiring the transmitted preamble 415 and the ensuing message
425 in the physical layer, the satellite access network 100
transmits AI signal 423 or 433 to the mobile station 170 and at the
same time, passes the received message 425 to the control station
150. The control station 150 transmits the acquisition indicator
signal to the mobile station 170 through the satellite access
network 100.
Referring to FIG. 4, the RACH 405 is an embodiment of the present
invention, in which the packet transmission time unit of the
preamble 415 and the message 425 becomes the radio frame 411 of the
control channel 401. The acquisition indicator signal of the RACH
405 is transmitted through the acquisition indicator channel
403.
In this embodiment of the present invention, as illustrated in the
RACH 405 and the acquisition indicator channel 403, the mobile
station 170 waits the time of T.sub.P (see 403) from the starting
point of the radio frame 411, in which the preamble 415 and the
message 425 are transmitted through the RACH 405, and then receives
the AI signals 423, 433 through the acquisition indicator channel
403 during the next time of T.sub.w (see 403).
Here, if the AI value (AI.sub.s) corresponding to the transmitted
preamble 415 and the message 425 is 0, it is a case that the AI
signal 423 or 433 is no acquisition indication. Since the mobile
station 170 does not receive the AI signal, the mobile station 170
regards the reception of the message 425 in the satellite access
network as failure and retransmits the preamble 415 and the message
425 in the next radio frame 411 immediately.
In case that the AI value (AI.sub.s) corresponding to the
transmitted preamble 415 and the message 425 is 1, the mobile
station 170 regards the reception of the message 425 at the
satellite access network as a success and waits for a response to
the message from the control station 150.
In case that the AI value (AI.sub.s) corresponding to the
transmitted preamble 415 and the message 425 is -1, the mobile
station 170 waits as long as a backoff time and attempts
retransmission of the preamble 415 and the message 425.
If the reception time T.sub.w (see 403) for the AI signal 423 or
433 is longer than the radio frame 411 and one or more AI value
AI.sub.s,j are received for a corresponding time unit, the multiple
numbers of AI values AI.sub.s,j are combined into one AI value
AI.sub.s, as expressed in Equation (9). In the embodiment of the
present invention, the mobile station 170 combines the received AI
value (AI.sub.s), when the mobile station 170 does not know exactly
in what radio frame, the acquisition indicator signal for the
mobile station 170 are transmitted.
.times..times..times..times..times..times..times..times..times..times..ti-
mes. ##EQU00002##
The RACH 409 of FIG. 4 is another embodiment of the present
invention, in which the packet transmission time unit of the
preamble 415 and the message 425 is the access frame 421. The
acquisition indicator signal of the RACH 409, is transmitted
through the acquisition indicator channel 407.
The RACH 409 and the acquisition indicator channel 407 are
time-aligned with each other based on the radio frame 411 of the
control channel 401 and the access frame 421 is the packet
transmission time unit.
The mobile station 170 waits the predetermined preamble-AI time
(T.sub.p-a) (see 407) from the starting point of the access frame
421, in which the preamble 415 and the message 425 are successively
transmitted through the RACH 409, and then it receives the
acquisition indicator (AI) signal 423 or 433 in the next access
frame of the acquisition indicator channel 407.
In this case, the mobile station 170 waits for the AI signal 423 or
433 to be received from the starting point of the sub-access frame
431 or 441 that correspond to the sub-access frame 431 or 441 of
the access frame 421 in which the latest preamble 415 and the
message 425 have been transmitted.
The mobile station 170 determines whether to retransmit the
preamble 415 and the message 425 according to the AI value
(AI.sub.s), i.e., AI signal 423 or 433.
If the AI value is 0 (no acquisition indication) or -1 (negative
acquisition indication), the preamble 415 and the message 425
should be retransmitted, the mobile station 170 retransmits the
preamble 415 and the message 425 in a predetermined
preamble-preamble-preamble time (T.sub.p-p)(see 407) from the
starting point of the access frame 421 in which the preamble 415
and the message 425 have been transmitted right before.
At this point, in case that the mobile station 170 should
retransmit the preamble 415 and the message 425, it uses the
sub-access frame 431 or 441 again that correspond to the sub-access
frame 431 or 441 selected for initial transmission, and then it
retransmits the preamble 415 and the message 425 based on the
sub-access frame 431 or 441.
The preamble-AI time (T.sub.p-a) (see 407) and the
preamble-preamble time (T.sub.p-p) (see 407) are set larger than
the sum of the maximum round trip delay and the signal processing
time between the mobile station 170 and the earth station 130.
Here, the signal processing time is the time consumed to receive
the preamble 415 in the satellite access network 100 (in case that
the AI signal 423 or 433 is used as the acquisition indication for
the preamble 415) or the time consumed to receive the preamble 415
and the message 425 (in case that the AI signal 423 or 433 is used
as the acquisition indication signal 423 or 433 for the message
425), and the time consumed in the satellite access network 100 to
determine if the preamble 415 or the message 425 have been received
successfully.
The values of the preamble-AI time (T.sub.p-a) (see 407) and the
preamble-preamble time (T.sub.p-p) (see 407) may be different
according to the associated satellite beams. The control station
150 broadcasts through the control channel 401 used in each of the
satellite beams.
The preamble-AI time (T.sub.p-a) (see 407) and the
preamble-preamble time (T.sub.p-p) (see 407) use the access frame
length as a basic unit (see 407). In FIG. 4, the values of the
preamble-AI time (T.sub.p-a) and the preamble-preamble time
(T.sub.p-p), are two and three access frames 421, respectively.
The preamble-AI time (T.sub.p-a) and the preamble-preamble time
(T.sub.p-p) are generalized and defined as Equation (10).
Preamble-AI time (T.sub.p-a)=(m+1).times.access frame
preamble-preamble time (T.sub.p-p)=(m+2).times.access frame
where m denotes an integer satisfying the below equation:
(m.times.access frame).ltoreq.maximum round trip delay time+signal
processing time<[(m+1).times.access frame]
FIG. 7 is a diagram describing a power increase retransmission and
retransmission in a random access channel (RACH) in accordance with
an embodiment of the present invention.
Before transmitting the message to the RACH 405, 409, the mobile
station 170 estimates the path loss of a corresponding link based
on the information on transmission power from the control channel
401 and the received power of the control channel 401 and
calculates the amount of power P.sub.est needed for the initial
RACH transmission. The actual transmission power P.sub.init has a
difference as much as the offset power .DELTA.P.sub.offset,i from
the power P.sub.est calculated as illustrated in FIG. 7. The power
of the retransmitted preamble and message is increased as much as
.DELTA.P.sub.step,i than the former transmission power, and the
retransmission using the increased power can be tried out up to
M.sub.ramp times. A retransmission period can be newly started
after the power increase period. In the retransmission period, the
initial transmission power is what has been recalculated from the
information and the received power of the control channel in the
present frame, and the retransmission can be tried out up to
M.sub.retx.
FIG. 8 is a flow chart showing an RACH access method in accordance
with an embodiment of the present invention.
The mobile station 170 receives information on the parameters below
from the satellite access network 100 through the control channel
401 before performing the RACH procedures of FIG. 8.
Among the parameters according to the RACH service class i are a
set of available spreading codes S.sub.pre,i for the RACH 405 or
409, a set of available signature codes C.sub.s used for the RACH
service class i, persistence test probability P.sub.i, initial
transmission power offset value .DELTA.P.sub.offset,i and
transmission power incensement value .DELTA.P.sub.step,i.
Also, as for common parameters, there are the maximum number of
retransmission cycles M.sub.retx, the maximum number of power
increase retransmissions M.sub.ramp, the range of maximum backoff
time T.sub.BO,min, T.sub.BO,max, the AI signal waiting time
T.sub.p, the AI signal reception duration T.sub.w, the waiting time
for acknowledgement reception T.sub.R, preamble-AI time T.sub.p-a,
preamble-preamble time T.sub.p-p, and maximum transmission time
offset T.sub.off,max.
For the above parameters for each RACH service class, by assigning
different values for different RACH service class, it is possible
to differentiate reception probability of the preamble and message
of different service classes.
However, parameters related to access process can be modified
according to the satellite mobile communication environment and the
selection of its system designer. Accordingly, it is obvious to
those skilled in the art that the parameters related to the access
process can be set up differently according to the satellite mobile
communication environment and the system designer, and the present
invention is not limited to the setup of the parameters related to
access process described above.
As shown in FIG. 8, at step S801, the mobile station 170 having
data to transmit initializes parameters related to the RACH 405 or
409 and selects a RACH service class according to the service type
of the message.
Subsequently, at step S803, the mobile station 170 initializes the
retransmission cycle counter m.sub.retx into 0. In the subsequent
access process, the retransmission cycle can be performed as much
times as the maximum number of retransmission cycles M.sub.retx. If
the retransmission cycle counter m.sub.retx exceeds the maximum
number of retransmission cycles
M.sub.retx(m.sub.retx>M.sub.retx) at step S805, the RACH access
attempt fails at step S807. For M.sub.retx retransmission cycles,
the parameters related to the RACH access are updated by using the
parameters received through the downlink control channel in every
retransmission cycle at step S809.
Subsequently, at step S811, a persistence test is performed. In the
persistence test, one number between 0 and 1 is generated randomly
and in case that the generated number is larger than the
persistence test probability P.sub.i, that is, the persistence test
is not satisfied, the processing waits at step S813 the next radio
frame 411, returns to S809 and repeats.
If the random number generated in the persistence test is equal to
or smaller than the persistence test probability P.sub.i, that is,
the persistence test is satisfied at step S811, a power increase
retransmission period begins and the power increase retransmission
counter m.sub.ramp is initialized into 0 at step S815. In each
retransmission cycle, the power increase retransmission can be
performed as much times as the maximum number of power increase
retransmissions M.sub.ramp.
During the retransmission process except the initial transmission,
if the mobile station receives a response corresponding to the
message 425 that it has transmitted before, it stops the RACH
access process of the present invention. This is the case that the
satellite access network 100 has successfully received the message
that the mobile station has transmitted through the RACH 405 or
409, and it has responded to the message through the control
channel.
If the mobile station does not receive the AI signal 423 or 433
after the transmission of the preamble 415 and the message 425, it
keeps retransmitting them. On the other hand, if the satellite
access network 100 receives the preamble 415 and the message 425
successfully, it transmits a response to the message 425 to the
mobile station 170. In this case, when the mobile station 170
receives the response to the message 425, it stops the RACH process
because it achieves its goal, even if the AI signals 423, 433 are
not received because of some errors.
In case that the power increase counter m.sub.ramp is larger than
the maximum number of power increase retransmissions M.sub.ramp at
step S817, the processing waits at step S819 the next radio frame
411, increases the retransmission cycle counter m.sub.retx by 1 at
step S821 and the process for a new retransmission cycle repeats
from the step S805.
In case that the power increase retransmission counter m.sub.ramp
is smaller than the maximum number of power increase
retransmissions M.sub.ramp at sep S817, the mobile station randomly
selects a signature of the available signature set for the selected
service class at step S823, and transmits the preamble 415 and the
message 425 at step S825.
In accordance with an embodiment of the present invention, in case
that the access frame 421, sub-access frame 431 or 441 and the
initial transmission time offset T.sub.off are applied, the mobile
station randomly selects an initial transmission time offset
T.sub.off of the range of -T.sub.off,max to T.sub.off,max as well
as a signature, an access frame and a sub-access frame at step
S823.
As an embodiment of the present invention, in case that the access
frame 421 is divided into a plurality of sub-access frames 431,
441, the mobile station randomly selects one of the sub-access
frames 431, 441 in the current access frame. In this case, the time
reference for the initial time offset is the starting time point of
the selected sub-access frame 431 or 441. As described above, a
spreading code S.sub.Pre can be set to distinguish the sub-access
frame and the RACH. The satellite access network 100 broadcasts the
spreading codes used for each sub-access frame and each RACH
through the control channel 401. If some preambles are transmitted
through the same RACH and the same sub-access frame 431 or 441,
they are spread by the same spreading code.
Subsequently, after waiting as much as the preamble-AI time
T.sub.p-a from the starting point of the access frame which is used
for the transmission of the preamble 415 and the message 425, the
mobile station receives the AI signal 423 or 433 on the acquisition
indicator channel 403 or 407. From the received AI signal, the
access status is determined (for the case that the access frame 421
and the sub-access frame 431 or 441 are applied). Or during the
time of T.sub.w (see 403) after waiting as much as T.sub.p (see
403) from the starting point of the radio frame which is used for
the transmission of the preamble 415 and the message 425, the
mobile station receives the AI signal 423 or 433 on the acquisition
indicator channel 403, and then checks the AI signal 423 or 433
from the satellite access network 100 (for the case that the access
frame 421 and the sub-access frame 431 or 441 are not applied.)
Here, in case that the access frame 421 and the sub-access frame
431 or 441 are not applied, the mobile station combines the AI
values AI.sub.s,j, corresponding to the signature used for the
preamble transmission through the acquisition indicator channel 403
or 407 , which are received during the time duration of T.sub.w
after the time of T.sub.p from the starting point of the radio
frame 411 in which the preamble 415 and the message 425 were
transmitted. The combined AI value AI.sub.s is determined as shown
in Equation (9).
If the AI value AI.sub.s is 1 (that is, the response is positive,
S827), it means that the preamble 415 (in case that the AI signal
423 or 433 is used as an acquisition indicator signal 423 or 433
for the preamble 415), or the preamble and the message (in case
that the AI signal 423 or 433 is used as an acquisition indicator
signal 423 or 433 for the message) is/are received in the satellite
access network 100 successfully. In this case, the mobile station
terminates the RACH access process, and waits for the response for
the message 425 from the satellite access network 100.
The RACH access process can be started again according to the
contents of the response for the-message transmitted from the
satellite access network 100.
In case that the AI value AI.sub.s is -1 (that is, the response is
negative, S829), the mobile station derives a backoff delay time of
the range of T.sub.BO,min to T.sub.BO,max, wait the derived backoff
delay time at step S831, increases the retransmission cycle counter
m.sub.retx by 1 at step S821, and repeats a new retransmission
cycle from the step S805.
When the mobile station 170 does not receive any AI signals on the
acquisition indicator channel 403 or 407, in other words, when the
AI value AI.sub.s is 0 (i.e., a case of no response, S829), after,
waiting (S833) the next radio frame 411, increasing (S835) the
transmission power by .DELTA.P.sub.step,i, and increasing (S837)
the power increase retransmission counter m.sub.retx by 1, the
mobile station repeats a new power increase retransmission period
from the S817.
According to the present invention described above, the probability
for successful packet reception is improved and the transmission
delay time is reduced. Particularly, in case that the physical
layer function of the satellite access network 100 is located in
the earth station, or that it is located in the satellite, the
waiting time for reception of the acquisition indicator signal
decreases remarkably. Also, the time alignment for the RACH
transmission and the AI signal between the mobile station and the
satellite access network can be carried out easily, and the
interference between the packets can be decreased by decentralizing
the reception time of preambles and message. Further, this
invention reduces the waiting time of a mobile station for
reception of the acquisition indicator signal by using the preamble
AI signal as a message AI signal.
While the present invention has been described with respect to
certain preferred embodiments, it will be apparent to those skilled
in the art that various changes and modifications may be made
without departing from the scope of the invention as defined in the
following claims.
* * * * *